Best unit for font-sizes in CSS

Question

What are the advantages & disadvantages of each? em, px, % and pt ?

My current choice are percentages, the only reason is because I can globally change the font-size of all elements, just by modifying the font size on the root element (body)

Answer 1

I would recommend EM — simply because I use Baseline CSS for my basic set up of forms, layout and most importantly type.

Can't recommend it enough : http://baselinecss.com/

Answer 2

My original design training said em's where possible.

I believe a main reason was that different browsers and devices have different resolutions, which is once again becoming a big issue.

I think it's frequently better to use em's and %'s as they are an abstraction layer particularly when compared to pixels. Both are similar in some respects as "100%" = "1em". Another problem with the pixel unit is that it does not scale upward for visually-impaired readers.
Today, for mobile, pad, etc. consideration it's often better to have specific stylesheets / rules for each one.

Also, for print concerns, em works well.

I have seen em unit called a standard for font sizes on the web, but the percent unit often gives a more consistent and accessible display. When user settings are changed, percent text scales well preserving readability, accessibility, and visual design.